lnwire: add new feature bits for explicit channel type negotiation

If these bits are present, then both sides can examine the new
CommitmentType TLV field that's present and use this in place of the
existing implicit commiment type negotiation. With this change, it's now
possible to actually deprecate old unsupported commitment types
properly.

feature/default_sets.go

@@ -60,4 +60,8 @@ var defaultSetDesc = setDesc{
 	lnwire.AMPRequired: {
 		SetInvoiceAmp: {}, // 9A
 	},
+	lnwire.ExplicitChannelTypeOptional: {
+		SetInit:    {}, // I
+		SetNodeAnn: {}, // N
+	},
 }